How Is Obsidian Formed?

Obsidian, a glass born of a volcano

Unlike most stones, obsidian isn't a crystal: it's a natural volcanic glass. It's classed as a mineraloid, because its material is amorphous, its atoms aren't arranged in an ordered lattice the way they are in quartz, but frozen in disorder, exactly as they are in window glass.

It's more than 70% silica (SiO₂), with a little aluminium, sodium and potassium. Its composition is therefore close to that of granite or rhyolite; what changes everything is its glassy texture, which comes from a very particular kind of cooling.

How obsidian forms

It all begins with an eruption of felsic lava, the so-called "acidic" kind, very rich in silica. That lava is extraordinarily viscous: it creeps along like thick honey. Inside it, long chains of silica tangle together and stop the atoms moving.

To form crystals, though, minerals need time and mobility. Here they get neither: the lava solidifies before any crystal can be born. The result is a dense, uniform glass with no crystalline structure, obsidian.

For a long time this was summed up as "flash cooling". More recent research complicates that story: to get a glass this dense and this free of bubbles, the cooling would in fact have to be slower than once assumed, over several weeks, long enough for the gas bubbles to dissolve away. One thing stays certain: it's the absence of crystallisation that makes all the difference.

Why is obsidian black?

Its dark colour comes mainly from small amounts of iron and magnesium scattered through the glass. Depending on the impurities and inclusions trapped as it flowed, obsidian can take on other appearances too:

• white flecks (cristobalite crystals) → snowflake obsidian;
• aligned gas bubbles creating a golden or silvery veil → gold sheen obsidian;
• magnetite nanoparticles that split the light → rainbow obsidian.

I set out all of these shades in the varieties of obsidian section of my complete guide, and in more depth in types of obsidian stones.

Where is obsidian found?

Obsidian forms wherever there have been silica-rich eruptions, often at the edges and surfaces of rhyolite flows, or around the margins of volcanic domes. It's on those edges, cooled faster and cleaner, that the finest material is found.

Among the famous deposits: Mexico, the United States (Glass Butte and the Big Obsidian Flow in Oregon, Obsidian Cliff in Yellowstone), Iceland (Mount Hekla), Italy (the Aeolian Islands, including Lipari), and also Armenia, Turkey, Japan and Greece. All of them volcano country.

Closer to home, the great source of this part of the world is Mayor Island, Tūhua, in New Zealand's Bay of Plenty. It was the largest single obsidian source in Aotearoa, and tūhua is itself the Māori word for obsidian. The stone was prized for cutting tools and traded across enormous distances, turning up at sites from the far south to the Kermadec Islands.

A stone that ages

A geological curiosity: obsidian is metastable. In other words, this glass isn't forever. Very slowly, across millions of years, it tends to reorganise itself into fine crystallisations, that's devitrification, and it's what produces the snowflake patterns.

Which is why there's almost no very old obsidian around: the oldest deposits have already transformed. Water speeds the process up, in contact with groundwater, obsidian gradually hydrates and turns into perlite, a duller glass. On the scale of the Earth, the shiny black stone you're wearing is rather young.

From raw rock to jewellery

The same material that once made blades and arrowheads, thanks to that conchoidal fracture with its razor edges, becomes jewellery today. The work is a matter of choosing good flakes, cutting them carefully, then polishing them until you reach that deep, almost liquid black.

In my studio, every stone is chosen one at a time, then set by hand. That attention is why no two pieces are quite alike. Have a look at my obsidian bracelets or browse the whole shop, everything is made in France and posted to Australia.

And if you're wary of tinted glass imitations, you're right to be: I explain how to spot a real obsidian in the guide.

Frequently asked questions